1. Field of the Invention
The present invention relates to an optical element having an anti-reflection function and optical scanning device using the optical element. In particular, the present invention is suitable for an optical system of an image forming apparatus such as a laser beam printer, a digital copying machine, or a multifunction printer, which has, for example, an electrophotographic process, and employs a structure in which a light beam emitted from a light source means is deflected by an optical deflector (deflecting means) to optically scan a surface to be scanned through an imaging optical system. The optical system includes an optical element which has an fθ characteristic and in which a subwave structural grating is provided, thereby recording image information.
2. Related Background Art
In a conventional scanning optical apparatus such as a laser beam printer (LBP), a light beam modulated in accordance with an image signal and emitted from a light source means is periodically deflected by an optical deflector composed of, for example, a polygon mirror, and converged to form a spot shape on a photosensitive surface of a recording medium by an imaging optical system having an fθ characteristic to optically scan the photosensitive surface thereof, thereby performing image recording.
FIG. 16 is a cross sectional view showing a principal portion of a conventional optical scanning device in a main scanning direction (main scanning cross sectional view).
In FIG. 16, a light source means 91 is composed of, for example, a semiconductor laser. A collimator lens 92 converts a diverged light beam emitted from the light source means 91 into a substantially parallel light beam. An aperture stop 93 adjusts a quantity of a light beam passing therethrough to shape a beam form thereof. A cylindrical lens 94 has predetermined power in a sub-scanning direction and images the light beam passing through the aperture stop 93 on a deflecting surface (reflecting surface) 95a of an optical deflector 95 (described later) with respect to a sub-scanning section to thereby form a substantially linear image.
The optical deflector 95 serving as a deflecting means is composed of, for example, a polygon mirror having four surfaces (rotary polygon mirror) and rotated at constant speed by a driving means such as a motor (not shown) in a direction indicated by an arrow A.
An imaging optical system 96 has a condensing function and an fθ characteristic and is composed of two imaging lenses, that is, first and second imaging lenses 96a and 96b. The imaging optical system 96 images the light beam based on image information, which is reflected and deflected by the optical deflector 95, on a photosensitive drum surface 97 serving as a surface to be scanned. The imaging optical system 96 achieves an optical face tangle error correction function by bringing the deflecting surface 95a of the optical deflector 95 and the photosensitive drum surface 97 into a conjugate relationship in the sub-scanning section.
In FIG. 16, the diverged light beam emitted from the semiconductor laser 91 is converted into the substantially parallel light beam by the collimator lens 92. The quantity of the substantially parallel light beam passing through the aperture stop 93 is adjusted to shape the beam form thereof, and is then incident on the cylindrical lens 94. The substantially parallel light beam incident on the cylindrical lens 94 exits therefrom without changes in the main scanning section and is converged and imaged on the deflecting surface 95a of the optical deflector 95 to form the substantially linear image (linear image extended in the main scanning direction) in the sub-scanning section. The light beam reflected on and deflected by the deflecting surface 95a of the optical deflector 95 is imaged to form a spot shape on the photosensitive drum surface 97 through the first and second imaging lenses 96a and 96b. At this time, the optical deflector 95 is rotated in the direction indicated by the arrow A, so that the photosensitive drum surface 97 is optically scanned at constant speed in a direction indicated by an arrow B (main scanning direction). Thus, image recording is performed on the photosensitive drum surface 97 serving as a surface of a recording medium.
In recent years, an imaging optical system of an optical scanning device has been normally made of plastic with which an aspherical shape is easy to form and produce. However, in view of technical aspects and cost, it is difficult to apply an anti-reflection coat to a lens surface of a lens made of plastic. As a result, Fresnel reflection is caused on the lens surface.
FIG. 17 is an explanatory graph showing angle dependencies of reflectances of P-polarized light and S-polarized light in a case where a light beam is incident on a resin optical member whose refractive index (n) is, for example, 1.524.
As shown in FIG. 17, in each of the cases where the P-polarized light and the S-polarized light both have an incident angle of 0, the reflectance thereof is substantially 4.4%. In particular, the amount of surface reflection of the S-polarized light on each optical surface (lens surface) becomes larger as the incident angle thereof increases. Therefore, light reflected on a lens surface on which the anti-reflection coat is not applied is reflected on another optical surface to finally reach the surface to be scanned, resulting in ghost.
In particular, as shown in FIG. 16, when a lens surface 96a1 of the first imaging lens 96a which is relatively closer to the optical deflector 95 than the second imaging lens 96b is a concave shape and a light beam is substantially perpendicularly incident thereon, Fresnel reflection light on the lens surface 96a1 returns to the optical deflector 95, reflected on the deflecting surface (reflecting surface) 95a of the optical deflector 95 to pass through the imaging optical system 96, and turns into ghost light as reaching the surface to be scanned 97. The ghost light thus generated leads to, for example, a partial density difference, which deteriorates an image outputted from an image forming apparatus.
Therefore, in order to solve the above-mentioned problem, the applicant of the present application has proposed an optical element in which a subwave structural grating called a 0th order grating is provided on at least one optical surface of the imaging optical system and a grating shape of a surface of the subwave structural grating is set as appropriate to thereby obtain an anti-reflection effect (Japanese Patent Application Laid-open No. 2003-185955).
On the other hand, there has been proposed a method of setting a pitch of an uneven shape formed in a light incident surface such that diffraction light other than 0th order diffraction light on each of an incident side and an outgoing side of light which is made random and has a critical wavelength or more is made to be substantially zero, thereby easily producing an anti-reflection structure (Japanese Patent Application Laid-open No. 2003-114316).
With respect to the pitch of the uneven shape, Japanese Patent Application Laid-open No. 2003-114316 discloses about a distance between grating vertices aligned in a one-dimensional direction. However, an arrangement of two-dimensionally arranged gratings is not sufficiently disclosed. Although the gratings are arranged at random, a variation allowable range of the random arrangement is not sufficiently disclosed. No problem pertaining to form birefringence is disclosed, either. Japanese Patent Application Laid-open No. 2003-114316 discloses only the normal occurrence of the form birefringence.
In general, a periodical subwave structural grating, in which a grating pitch becomes small and thus is substantially equal to or shorter than a wavelength of light and has anisotropy in an arrangement direction, shows form birefringence in some cases. This corresponds to, for example, the case where a shape of a grating, in which subwave grooves are aligned in a first direction set along a groove, such as a wave grating, is significantly different from that in a second direction orthogonal to the first direction.
According to “Principle of Optics III”, Tokai University Press, p 1030, when optically isotropic materials are regularly arranged as particles, each being sufficiently larger than a molecule and smaller than a wavelength of light, the optically isotropic materials act to cause the form birefringence. That is, a model such as a thin parallel plate assembly having periodicity of the order of wavelength or less as described in “Principle of Optics III” becomes a uniaxial crystal in which an effective dielectric constant obtained from a dielectric constant of a medium of a plate portion and a dielectric constant of a medium of a non-plate portion behaves differently on an electrical vector parallel to the plate portion and an electrical vector perpendicular to the parallel portion.
In other words, the subwave structural grating, whose grating pitch is substantially equal to or shorter than the wavelength of light, shows a dielectric constant differently on two axes, that is, a grating arrangement direction and a direction perpendicular to the grating arrangement direction, according to a direction of a polarization plane of an incident beam. However, the grating which shows the dielectric constant differently on the two axes, that is, the grating arrangement direction and the direction perpendicular to the grating arrangement direction, cannot obtain a sufficient anti-reflection effect.
For example, when a light beam is perpendicularly incident on an optical element, an anisotropy medium shows the same transmission and reflection properties on two polarized beams orthogonal to each other. However, a form birefringent element or a uniaxial crystal shows the transmission and reflection properties completely different on the two polarized beams orthogonal to each other.